The Harderian gland (HG) of the Indian palm squirrel, F. pennunti, is composed of acini of a single type of simple columnar cells with uniform-sized lipid droplets and porphyrin (PI in the lumen. Morphologically it presented no sexual dimorphism except for the HG weight which revealed that males are acyclic. Circadian study of Harderian gland porphyrin (HG-P), plasma melatonin, (aMT) and testosterone showed a characteristic two peak cyclicity. In females, HG, HG-P and pineal gland weight, and plasma aMT presented an annual inverse relation. Circadian study in females only exhibited a two peak cyclicity of HG-P, plasma aMT, and estradiol. Pinealectomy (PX) and harderianectomy (HGX) revealed increased HG weight and gonad weight in males . Gonadectomy (GX), on the other hand, had no effect on HG in males. PX in females brought almost a similar effect as noted for males, but HGX had no effect on ovarian weight. GX, interestingly, reduced HG weight and P concentration. Daily evening (4:30-5:OO) administration of aMT and 5-methoxytryptamine (5-MT) in males reduced HG weight and HG-P content only in aMTtreated male and female squirrels, thereby suggesting that HG-P is perhaps negatively regulated by pineal gland production and vice versa. Injections of gonadotropin and steroids during the sexually inactive phase showed no effect on HG-P content in both sexes. Short photoperiod (SP) in both sexes stimulated pineal weight without affecting HG weight, while long photoperiod (LP) increased HG-P but reduced the plasma aMT level again without affecting HG-P content. Continuous dark (CD) decreased HG-P, whereas continuous light was ineffective without effecting HG weight in both sexes. In conclusion, HG in this rodent is functionally an important gland having diverse physiological effect in both sexes sometimes with a very clear HG-pineal-gonad relationship. o 1996 Wiley-Liss, Inc.